Britain and the US are jointly hacking Yahoo and Google data around the world, according to the latest leak from former security worker Edward Snowden.

The Washington Post reports that America's National Security Agency and Britain's GCHQ are copying entire data flows across fibre-optic cables that carry information between the data centres of the Silicon Valley giants.

According to a secret record dated January 9, 2013, the NSA sends millions of records every day from Yahoo and Google internal networks to data warehouses to its headquarters.

In the last 30 days, the report on the Post website said, collectors had processed and sent back more than 180 million new records - ranging from "metadata," which would indicate who sent or received emails and when, to content such as text, audio and video.

The NSA's principal tool to exploit the data links is a project called MUSCULAR, operated jointly with GCHQ.

White House officials and the Office of the Director of National Intelligence, which oversees the NSA, declined to comment, the Post said.

Google said it was "troubled by allegations of the government intercepting traffic between our data centres, and we are not aware of this activity."

At Yahoo, a spokeswoman said: "We have strict controls in place to protect the security of our data centres, and we have not given access to our data centres to the NSA or to any other government agency."